1. Field of the Invention
The present invention relates to a high-repetition-rate femtosecond regenerative amplification system, and more particularly, to a femtosecond regenerative amplification system which can amplify pulses at a high repetition rate of about 100 kHz and produce pulses of a high energy of tens of microjoules (μJ) and a gigawatt (GW) peak power.
2. Description of the Related Art
Lasers have found innumerable applications using their characteristics such as monochromaticity, directionality, and brightness. From the beginning, researches have sought to increase the peak power of their lasers through Q-switching, mode-locking, or amplification. However, this quest was blocked since high intensities were found to affect the optical properties of lasing materials and optical components. Ultra-short high-intensity lasers eliminating the intensity barrier have recently been developed by combining chirped pulse amplification (CPA) technology with ultra-short (picosecond or femtosecond) pulse technology. Such ultra-short high-intensity lasers are high power laser systems which can amplify ultra short pulses with a femtosecond or picosecond pulse width to provide terawatts or more of power. The femtosecond laser technology based on Ti:sapphire gain medium has make it possible to produce pulses having pulse durations of 5 femtoseconds or peak powers of petawatts (PW=1015 W).
In particular, since the femtosecond laser technology can obtain a high peak power with a low energy, thermal damage can be reduced and amplification can be acquired at a high repetition rate of 1 kHz or more. FIG. 1 is a graph illustrating a relationship between peak power and repetition rate in femtosecond lasers using titanium sapphire (Ti:S) amplifiers. When lasers have a repetition rate of about 10 Hz, the lasers generally have peak powers of a few terawatts (TW) to tens of terawatts. If lasers have a repetition rate of 1 Hz or less, they can have peak powers of petawatts (PW). When lasers have a repetition rate of about 1 to 10 kHz, the lasers generally have peak powers of sub-terawatts (sub-TW) corresponding to approximately 0.1 to 0.5 TW, but can have peak powers of terawatts (TW) as well. When lasers have a high repetition rate of 100 to 250 kHz, the lasers generally have peak powers of tens of megawatts (MW), but can have peak powers of sub-gigawatts (sub-GW) less than 0.2 GW as well.
When the repetition rate increases to kHz, a necessary pump laser energy decreases, which is advantageous to the manufacturing of compact femtosecond lasers. Since the lasers have a high working rate, they can be used in X-ray generation experiments as well as industrial applications such as micro-machining and glass cutting. However, compared to 1 to 10 kHz femotosecond lasers, 100 kHz lasers have a very low output power. As a result, commercial lasers of ˜100 kHz repetition rate have not been frequently used so far. A pulse switching method, a pump source, and an output energy according to a laser repetition rate are shown in the following table. Here, the output energy is obtained when a single stage amplifier is used.
TABLE 1Pulse switchingRepetition ratemethodPump sourceOutput energy10HzElectro-optic10 ns Q-switched~10mJmodulationgreen laser(Nd:YAG)1–20kHzElectro-optic100–300 ns~1mJmodulationQ-switched greenlaser (Nd:YLF)100–250kHzAcousto-opticContinuous wave~1μJmodulation(CW) green laser(Nd:YVO4, Argon)
In Table 1, lasers having a 100-250 kHz repetition rate generally deliver an output energy of 1 μJ or so, and the highest output energy to date at a 100 kHz repetition rate is 7 μJ.
FIG. 2 is a block diagram of a conventional 100 kHz laser system. The conventional 100 kHz laser system includes a Ti:S oscillator 10, a Ti:S regenerative amplifier 20, and a compressor 30. A CW argon laser 13 pumps the oscillator 10 and the regenerative amplifier 20. A part of a beam output from the CW argon laser 13 and separated by a first beam splitter 15 travels toward the regenerative amplifier 20, and the rest of the beam is reflected by a reflective mirror 17 to the oscillator 10. For example, the oscillator 10 outputs 100 pJ, 75 femtosecond laser pulses, the regenerative amplifier 20 amplifies the laser beams and outputs 1.87 μJ, 10 picosecond pulses, and the compressor 30 compresses the pulses and outputs 1 μJ, 130 femtosecond pulses.
FIG. 3A is a plan view of the regenerative amplifier 20 used in the conventional 100 kHz laser system of FIG. 2. Referring to FIG. 3A, pulses output from the oscillator 10 pass through a polarizing beam splitter 41, a Faraday rotator 43, a half wave plate 45, and a reflective mirror 47 and then are input to the regenerative amplifier 20. The regenerative amplifier 20 includes a resonator 21 for resonating pulses, an acousto-optic modulator 22 using a Bragg cell called a resonator dumper for switching, a Ti:S gain medium 23, and a Q-switch 24. The Q-switch 24 is separately provided from a laser pulse resonance switch when a CW laser is used as a pump source.
The resonator 21 includes a plurality of curvature mirrors CM1, CM2, CM3, and CM4, such that the pulses output from the oscillator 10 are amplified while repeatedly reciprocating between the reflective mirrors. Here, the pulses are naturally stretched due to positive dispersion whenever the pulses pass through the Ti:S gain medium 23 or the acousto-optic modulator 22 using the Bragg cell. The stretched and amplified pulses are compressed by the compressor 30 to reduce a pulse width and increase a peak power.
FIG. 3B is a plan view of the compressor 30 used in the conventional 100 kHz laser system of FIG. 2. Referring to FIG. 3B, the compressor 30 includes first and second prism sets 32 and 34 each including a plurality of prisms. A planar mirror 31 extracts a beam by being moved up and down.
Since the conventional laser system constructed as shown in FIGS. 3A and 3B uses the CW laser as the pump source, the conventional laser system has a weak pump intensity, thereby lowering an amplified energy. Also, the conventional laser system requires the Q-switch 24 in addition to the modulator 22 in the regenerative amplifier 20, thereby increasing the number of components. Further, the compressor 30 includes the plurality of prisms and distances between the prisms are wide, thereby increasing the size of the conventional laser system. Furthermore, remaining high order dispersion degrades a compression ratio, thereby making it difficult to produce short pulses less than 100 femtoseconds.
On the other side, the conventional 100 kHz laser can efficiently amplify laser pulses using a CPA scheme that stretches and amplifies pulses and then compresses the amplified pulses. FIG. 4 illustrates pulses stretched, amplified, and compressed by a CPA scheme used in the conventional laser system of FIG. 2. Referring to FIG. 4(a), pulses are output from the oscillator 10. Referring to FIG. 4(b), the pulses are provided with positive dispersion by a pulse stretcher including a pair of diffraction gratings to be stretched to tens of picoseconds. At this time, the stretched pulses are positively chirped from low to high frequency such that the frequency of a front end of the stretched pulses is less than the frequency of a rear end of the stretched pulses. Referring to FIG. 4(c), the stretched pulses are amplified by the amplifier 20. Referring to FIG. 4(d), the amplified pulses are compressed by the compressor 30. In the typical CPA scheme, the diffraction gratings are used in both pulse stretching and compression modes. In detail, during the pulse stretching mode, the diffraction gratings are arranged to provide positive dispersion, and during the pulse compression mode, the prisms and the diffraction gratings are arranged to provide negative dispersion. The CPA scheme can reduce the pulse width through effective dispersion compensation. However, the CPA scheme has disadvantages in that because the pluses pass four times through the diffraction gratings during the pulse compression mode, compression efficiency is lowered to 60% or less. Since the 100-kHz CPA laser still uses the CW laser as a pump laser, the output energy is still low, too.